This invention relates to a starch-based, formaldehyde-free adhesive composition suitable for use in the preparation of paperboard. The adhesive of the present invention may be used to provide formaldehyde-free corrugated paper products which possess a high degree of water resistance.
As used herein the term "corrugated paperboard" refers to a fluted medium and a facing adhesively joined to the tips on one or both sides of the fluted medium.
The procedures employed in the production of corrugated paperboard usually involve a continuous process wherein a strip of paperboard is first corrugated by means of heated, fluted rolls. The protruding tips on one side of this fluted paperboard strip are then coated with an adhesive, and a flat sheet of paperboard, commonly known in the trade as a facing, is thereafter applied to these tips. By applying heat and pressure to the two paperboard strips thus brought together, an adhesive bond is formed therebetween. The above-described procedure produces what is known to those skilled in the art as a single-faced board in that the facing is applied to only one surface thereof. If a double-faced paperboard is desired, in which an inner fluted layer is sandwiched between two facings, a second operation is performed wherein the adhesive is applied to the exposed tips of the single-faced board and the adhesive-coated tips are then pressed against a second facing in the combining section of the corrugator under the influence of pressure and heat. The typical corrugating process and the use or operation of corrugators in general are described in U.S. Pat. Nos. 2,051,025 and 2,102,937 issued on Aug. 18, 1936 and Dec. 21, 1937, respectfully to Bauer, both incorporated herein by reference.
The particular adhesive employed in the corrugating process is selected on the basis of several factors, including the type of bond required in the final application of the finished corrugated product. Starch-based adhesives are most commonly used due to their desirable adhesive properties, low cost and ease of preparation.
The most fundamental of the starch-based corrugating adhesives is an alkaline adhesive which is comprised of raw ungelatinized starch suspended in an aqueous dispersion of cooked starch, described in U.S. Pat. No. 2,102,937 to Bauer, incorporated by reference above. The adhesive is produced by gelatinizing starch in water with sodium hydroxide (caustic soda) to yield a primary mix of gelatinized (or cooked) carrier, which is then slowly added to a secondary mix of raw (ungelatinized) starch and water (and, optionally, borax) to produce the full-formulation adhesive. In the corrugating process, the adhesive is applied (usually at a temperature between 20.degree. and 55.degree. C.) to the tips of the fluted paper medium or single-faced board, whereupon the application of heat causes the raw starch to gelatinize, resulting in an instantaneous increase in viscosity and formation of the adhesive bond. However, such adhesives are very water sensitive and the bonds formed often fail when the corrugated board is wet.
Because it is often desired or necessary in the manufacture of corrugated paperboard that the adhesive yield water-resistant bonds which can withstand extended exposure to high humidity, liquid water, melting ice and the like, a number of approaches have been devised to produce water-resistant corrugating adhesives. One method involves the preparation of an acidic, starch-based adhesive wherein urea-formaldehyde resin is added to the composition, together with an acidic catalyst such as aluminum sulfate, to produce water-resistant bonds in the corrugated board manufactured therewith. This adhesive composition itself, however, is deficient in other important properties such as corrugator bonding speeds (since the bond forms slowly under acidic conditions), viscosity stability, and pot life and exhibits excessive formaldehyde odor due to the liberation of formaldehyde from the cured adhesive. In addition, acidic corrugating adhesives tend to be corrosive.
The many disadvantages associated with the acidic corrugating adhesives prompted the development of water-resistant alkaline curing starch-based adhesives for use in the corrugating industry. In the preparation thereof, a thermosetting resin, such as, e.g., urea-formaldehyde, resorcinol-formaldehyde, melamine-formaldehyde, phenol-formaldehyde, diacetone acrylamide-formaldehyde, ketone-aldehyde and urea-acetone-formaldehyde condensate, has been added to the adhesive as a crosslinking additive for the amylaceous components to produce water-resistant bonds. Such adhesives are quite stable and can be run at high corrugator speeds.
However, as the scientific community is becoming aware of the hazards of formaldehyde, efforts have been accelerated to reduce exposure of the public, both in the home and workplace. Thus, the presence of formaldehyde in these adhesives makes them unsatisfactory for use in corrugated board, particularly in board used in applications, such as food packaging.
In response to this, a number of researches have tried to reduce or eliminate formaldehyde in corrugating adhesives. For example, U.S. Pat. No. 4,400,480 discloses an adhesive which contains an acetone-formaldehyde condensate low in free formaldehyde. U.S. Pat. No. 4,775,706 discloses formaldehyde-free corrugating adhesives containing a crosslinking agent comprising a halohydrin quaternary ammonium monomer. Also, Japanese Patent Laid-Open Sho-55-54371 discloses a starch paste crosslinked with a polyamide epihalohydrin condensate, copolymerized with an ethylenically unsaturated monomer.
The above methods, however, can result in undesirable adhesives which still contain formaldehyde (in smaller quantities). Further, in the cases where formaldehyde is eliminated entirely, the adhesives often require the crosslinking agent to be a major component, making it undesirable in many applications.
Accordingly, it is an object of this invention to present a formaldehyde-free corrugating adhesive which cures under alkaline conditions and exhibits a satisfactory degree of water resistance. It is further an object of this invention to present adhesives which can be obtained easily and do not require the use of large quantities of the crosslinking agent.